WEll, given that Both countries have run or are running a “Command” Economy, where anything connected with Military is the Purview of the State, I doubt that any large amount of “Primers” would have been available before the Cold War; Maybe from Soviet Russia, to "like-minded socialist gov’ts ( ie, Republicans in Spain, during SCW) but in any case, they would be in “Factory Packs” (Large trays holding up to 1,000 (Berdan) primers, buffered by wooden or Paper separators). China in particular, had to keep up with its own internal use during the 20s and 30s; and during the Civil War and early Mao Period, it was all local use for Military Purposes.
Nowadays, the Russians do export (PMC with Murom initially, then “Tula and Wolf” and even Murom themselves, after they gave PMC the Bum’s rush. As to China, Norinco does sell shotshells and Metallic ammo overseas, but I have not seen any Primers or Powder from them that wasn’t “Gov’t to Gov’t sales”. Even if a Budding entrepreneur did get a Primer Sales agreement with Norinco, the USA is Out, due to the Clinton Embargo ( ditto the Russians under Obummer)